A report is given of observations of the effects of soil conditions on damage in Mexico City and Guerrero State in the earthquake of July 28, 1957. An isoseismal map is presented which identifies anomalies in intensity distribution which appear to be closely related to local geology and soil conditions. Comparison is made between Mexico City, 170 miles from the epicenter and founded on the deep alluvium of Lake Texcoco, and several cities and villages 60 miles from the epicenter, founded on firmer deposits or granite.
Mexico City suffered Modified Mercalli intensities of VII and IV, respectively, in that part of the lake-bed area where tall buildings stand, and on the more compact formations. Of the several cities 60 miles from the epicenter, all suffered intensity V or less except Chilpancingo, where the intensity was VII to VIII. Chilpancingo rests on deep unconsolidated deposits; the other places are on rock or shallower unconsolidated deposits.